1. Field of the Invention
The present invention relates to an apparatus for adjusting light beam direction of progress by making use of the relation between the incident angle and the deviation angle of a prism.
2. Prior Art
The angle between the incident ray and the outgoing ray observed when monochromatic ray of light pass through a prism is regarded as deviation angle. The deviation angle becomes minimum when the incident angle is equal to the outgoing angle and, in an area around the minimum value, the rate of change in the deviation angle with respect to the variation of incident angle is low.
As shown in FIG. 13, a prism having an apex angle of .omega. is disposed so that rays of light strike the incident face of the prism perpendicularly. If this prism is rotated clockwise or counterclockwise in the figure, the relation between the rotational angle .theta. of the prism (in FIG. 13, the clockwise direction is designated "+" and the counterclockwise direction is designated "-") and the deviation angle .theta.' behaves as shown in FIG. 14. In FIG. 14, the relation for .omega.=10.degree. is plotted by a continuous line and that for .omega.=5.degree. is plotted by a broken line.
The smaller the apex angle, the lower the rate of change in the deviation angle and the rate of change is extremely lowered particularly in the vicinity of the minimum angle of deviation denoted in FIG. 14 by arrows. By making use of such characteristics, the angle of the outgoing rays can be changed in a minute level while adjusting the incident angle of rays of light with respect to the prism in relatively large level.
However, if such an adjustment is effected using a single prism, there is necessarily formed an angle deference between the incident rays and the outgoing rays irrespective of the angle between the incident rays and the prism For this reason, the cross-sectional shape of incident light beam differs from that of outgoing light beam. Moreover if light of which wave length distribution extends over a wide range, such as white light is used as an incident light, the chromatic aberration cannot also be neglected. Therefore, such adjustment cannot be employed in certain applications.
These problems can be solved by using an apparatus for adjusting light beam direction shown in FIG. 15, in which two prisms are utilized.
As shown in FIG. 15, the incident faces and the outgoing faces of prisms 1 and 2 constituting the apparatus are parallel with an axis which is perpendicular to the incident direction of light beam. In this apparatus, the direction of outgoing light beam is changed by rotating the prism 1 around a rotational axis L.sub.1 parallel to the axis, in accordance with the aforementioned relation between the incident angle and the deviation angle. According to the apparatus having the construction shown in FIG. 15, it is possible to attain a state in which the angle difference between the incident light beam and the outgoing light beam is zero (this state is defined as "zero-adjusting state") and in such case, the alternation of the cross-sectional shape and the chromatic aberration can be eliminated. However, since the incident light beam and the outgoing light beam cause a shift therebetween even if the incident and outgoing light beam are parallel to each other, it is quite difficult to properly arrange optical systems at the incident side and the outgoing side of the prisms.
One example of an apparatus for 2-dimensionally adjusting light beam direction utilizing two prisms is shown in FIGS. 16-18.
As seen from these figures, the apparatus is provided with two prisms 3 and 4. The prism 3 is designed to be able to rotate around a rotational axis L.sub.2 parallel to an axis z while the prism 4 is designed to be able to rotate around a rotational axis L.sub.3 parallel to an axis y. This apparatus makes it possible to 2-dimensionally effect fine adjustment of the angle of outgoing rays by controlling the rotational angle of the prisms 3 and 4. In this connection, FIGS. 17 and 18 are diagrams illustrating how the prisms shown in FIG. 16 refract rays of light. FIG. 17 is a sectional view taken along a plane passing through the center of the prisms and parallel to x-y plane and FIG. 18 is a sectional view taken along a plane passing through the center of the prisms and parallel to x-z plane.
However, if the apparatus shown in FIG. 16 is used, it is also impossible to solve the problems of the formation of the angle difference and the chromatic aberration in each direction, as in the apparatus shown in FIG. 13 in which a single prism is employed.